Panel heaters are now applied to various uses. In particular, panel heaters using PTC thermistors as heating elements are free from overheating, and hence enjoy advantages such as automatic self control of calorific value even under change of surrounding temperature. Accordingly, they are used as components for general use heaters, such as food warmers for hospitals, and for numerous other purposes. Technology of this type is disclosed as panel heaters in Japanese Unexamined Patent Publication No. 61-256123 and Japanese Unexamined Patent Publication No. 62-16994.
The panel heaters as disclosed in the aforementioned unexamined published Japanese patent applications, however, suffer problems. Those problems include the fluctuation of temperature due to local heat just above the heater; insufficient heat conduction from the heater element to the panel due to the presence of thermal resistance at the joint; the necessity of additional process steps, such as bolting, for the joints; and the need for a complicated structure of reinforcing materials which are incorporated into the heater to support the heavy load being applied from the upper side of the heating element.
Furthermore, on installing a panel heater comprising a combination of a PTC thermistor and a metallic heat radiation sheet to materials for walls and floors, a PTC thermistor sintered element capable of covering a large area is a requisite. In practice, however, large area panels are unfeasible, because such a large sintered element for a PTC thermistor will require great difficulties in manufacturing. Even if an alternative process for realizing a large area panel heater were to be taken, i.e., integrating smaller PTC thermistor sintered elements into a larger one, the process would be uneconomical and energy-insufficient. Moreover, such a sintered element obtained by integrating smaller pieces of PTC thermistor sintered elements is not practical, because a large heat emission occurs preferentially at the joint portions of the PTC thermistors with a metallic sheet.
An advantage of a PTC thermistor is a quick temperature rise which is realized as a consequence of the rush current (a large initial surge of current which is generated immediately after applying the current). However, the rush current from a plurality of PTC thermistors will accumulate into a large current that has unwanted effects such as activation of the breaker.
An object of the present invention is to overcome the prior art problems as mentioned hereinbefore, and to provide a simply structured panel heater resistant to heavy loads, and free from local overheating.
Because panel heaters are characterized by their thin sheet-like structure, they have been utilized as general use heaters, floor heating, etc. Accordingly, the PTC thermistor heaters tend to be used more frequently because they have a self control function for temperature, which provides enhanced safety.
Conventional PTC thermistor heaters for use in floor heating were planar heaters comprising a heat-resistant insulating organic material having conductive materials such as carbon particles dispersed therein.
Those conventional planar heaters, however, suffered the following drawbacks: (1) Lack of stability in heat emission--the contact state of the conductive material particles changes along with the changing temperature, providing no assurance that the initial contact state will recover upon return to the initial temperature; (2) Non-uniform heater temperature--as mentioned above, because the contact state of the conductive particles is non-uniform, the electric resistance differs from one place to another; and (3) Difficulty in placing the panel heater--because virtually the entire front panel is covered with a heating element, the positions for safely nailing up the panel are highly restricted.
Accordingly, a second object of the present invention is to provide a panel heater which stably emits heat without suffering non-uniform temperatures, and which can be freely fixed to a floor using a nail or the like. Furthermore, it is also an object of the present invention to shorten the transient time from when the electric source is turned on to the point the rush current reaches the panel heater.
A PTC thermistor has a low initial resistance, and it is known that a large rush current generates on it when it is used as a heating element. Accordingly, a larger current capacity is required for the initial stage than that required for the stationary state. This signifies that, when a PTC thermistor is applied to panel heaters or other uses which require a large output, the output and the number of panel heaters must be confined to a certain range.
To cope with the above problems, much effort has been put into reducing the rush current to as low a value as possible.
Such efforts are described in Japanese Unexamined Patent Publication No. 55-97143, in which a PTC thermistor being serial connected with a negative temperature coefficient thermistor is disclosed, or in Japanese Unexamined Patent Publication No. 54-115443, in which an ohmic contact being connected with a non-ohmic contact is disclosed. Furthermore, Japanese Unexamined Patent Publication No. 49-27932 discloses a combined use of PTC thermistors differing in Curie point, and Japanese Unexamined Patent Publication No. 63-218184 discloses the use of a phase temperature control device.
The conventional techniques as cited above are disadvantageous in that they incorporate additional process steps that make the circuit more complicated. In particular, panel heaters and the like as disclosed in Japanese Unexamined Patent Publication No. 49-27932 suffer from considerable temperature fluctuation.
A third object of the present invention is, therefore, to control the rush current by the thermistor itself without using any additional circuits and elements.
Heating elements obtained by joining and electrically connecting a plurality of PTC thermistors with two electrodes are used practically in such items as fan heaters, hair driers, and bedding driers. The output power of a device using such a heating element is controlled by the placement of a plurality of those heating elements each composed of PTC thermistors joined and connected with two electrodes, and thus applying the current to only the selected heating elements. The output power is thus controlled by the number of heating elements to which the current is applied.
However, to increase the output in heating elements of the above type, a larger number of heating elements must be incorporated. The installation of these additional elements will require not only more space, but higher cost as well.
A fourth object of the present invention is, therefore, to provide a PTC thermistor heater composed of a plurality of PTC thermistors which are integrated into a single heating element, capable of changing the output by itself.
A fifth object of the present invention is to avoid loss of strength of the PTC thermistor heater due to the spaces which results from the PTC thermistor elements being arranged leaving spaces in the heating element, and also to prevent fire or breakage from occurring due to a short circuit.